Forward looking infrared (FLIR) imaging systems incorporate photodetector arrays of photosensitive semiconductor materials such as Hg.sub.1-X Cd.sub.X Te which can be formulated to respond to wavelengths in the 3-5 and 8-12 micrometer atmospheric windows. Both p-n junction photodiode arrays and MIS photocapacitor arrays of Hg.sub.1-X Cd.sub.X Te provide satisfactory focal plane imaging in the infrared regime. These systems produce standard video pictures wherein the thermal temperatures of the viewed scene are displayed as various shades of gray.
Continued efforts to improve the performance of FLIR systems frequently result in new advances for photodetector arrays as well as modifications to image processing circuitry and introduction of additional system components. Field implementation of these improvements often presents design difficulties. This is because FLIR systems are complex and costly electro-optical/mechanical assemblies that must be individually configured to meet the physical constraints of each application. For example, although different systems may be formed of many common modules, the systems may be incompatible because they are arranged for different space geometries or because they each require optical inputs from different directions. In addition, performance requirements, e.g., field of view, magnification and resolution, which affect the overall size of a system, will generally differ.
Implementation of FLIR system improvements may occur when a complete system is designed for a new application, or, as part of an effort to upgrade an older system. When cost effective it is desirable to upgrade or retrofit older systems rather than to replace them. However, the redesign necessary to incorporate an improvement into an older system may require substitution of more costly components which are, at most, only indirectly related to the improvement.
In addition to incorporation of more advanced detector arrays, recent improvements in the performance of FLIR systems have required the provision of thermal reference sources as well as circuitry to perform DC restoration and detector electronic gain balance functions. When upgrades to scanning FLIR systems include these improvements it is necessary to modify the optical path so that the detector array can view the thermal source. This can be accomplished by replacing either the imaging optics or the afocal optics with a lens system that includes an additional image plane such that the reference source can be introduced during inactive scan time, i.e., when the scene is not being viewed. In the past each of these approaches has required substantial optical or mechanical modification to the system.
Most FLIR systems do not include reimaging imagers and additional space would be required to replace an imager with a reimaging imager. Furthermore, a thermal reference source cannot simply be positioned in the intermediate image plane of a reimaging imager. This is because FLIR detectors are not designed to scan such image planes. Thus, in order to view the thermal source apart from the radiation which forms an image of the scene, it is necessary to introduce an optical/mechanical chopper wheel within the optical path. For example, in order to periodically provide an image of the thermal source to the detector array a rotating mirror could be synchronized and phased with the scanner. With this arrangement an image of the thermal source is provided to the detector array during periods when the scanner is not providing image data to the detector array. An additional mirror may be required to complete the optical path, particularly if an existing system is being upgraded to include the thermal reference source. Introduction of the mirrors, as well as an electromechanical system to effect synchronous rotation, can be a difficult and costly task, especially in existing systems, because of severe space constraints and the requirements for moving parts and circuitry for effecting synconous chopping.
Alternately, a reimaging afocal may be introduced in lieu of a Galilean afocal to provide an image plane for the introduction of thermal sources in front of the scanner. By replacing the Galilean afocal with a reimaging afocal lens system the thermal sources can be scanned along with the field of view. However, different FLIR systems often require different fields of view or different afocal levels of magnification. Thus introduction of the thermal reference source would require redesign for each distinct application. Furthermore, reimaging afocals are considerably more expensive and require more space than Galilean afocals.
In summary, known methods for introducing thermal reference sources into FLIR systems are costly and require custom modifications for dissimilar systems. It would be advantageous to provide a means for incorporating thermal reference sources into both new and existing FLIR systems which is simpler and less costly than known techniques.